MXS-1220: Move header generation back to HttpResponse

The actual list of headers is not known when the request is first
generated. This prevents the headers from being generated in admin.cc
which handles things on a lower level.

The moving of the header generation is done with the OPTIONS method in
mind. This header needs to be generated inside the RootResource class
which manages the navigation of the resources.

server/core/maxscale/httprequest.hh

@@ -119,7 +119,7 @@ public:
      *
      * @return Raw JSON body or NULL if no body is defined
      */
-    json_t* get_json()
+    json_t* get_json() const
     {
         return m_json.get();
     }